This invention relates to the ice disaggregating arts and, more particularly, to aspects of the ice engaging teeth and of the means by which supported upon a large, vertically oriented drum adapted to engage and disaggregate ice.
In the petroleum exploration and production industry, it is often necessary to move and station men and equipment in relatively hostile environmental regions. In recent years, the emphasis on oil production from the far north has necessitated the development of new techniques for encountering formations of encroaching ice floes and the movements thereof which threaten the stability and/or position of equipment situated therearound.
In the Arctic, large offshore regions are often covered by thick layers of ice. Currently, there is considerable activity in this and other frozen areas directed toward the discovery and development of sources of petroleum and other natural resources. The search for and production of these resources require operational platforms for housing equipment and personnel. These platforms are manually transported to their operational sites and are maintained in a relatively fixed position with respect to the underwater floor by anchoring thereto and/or the utilization of dynamic positioning techniques. It may be noted, however, that some such platforms are self-propelled. In the normal course of operation, pipes are extended from the platform into the earth's sub-surface for the recovery of natural resources such as petroleum. It is thus important to maintain the platform within a predetermined envelope in order to prevent breaking or withdrawing the pipe from the earth.
Platforms located in both shallow and deep covered waters are exposed to ice floes which sometimes float freely on the water and/or a unitary ice mass which flow insidiously. The ice may be comprised of such mass that a platform is susceptible to damage or destruction as a result of forces imparted thereagainst by the moving ice. The Arctic Ocean, for example, is characterized by air temperatures ranging from -70.degree. F. to 70.degree. F., ice sheets and thicknesses between 6 and 10 feet, and pressure ridges of 10 to 100 feet. In such conditions, ice typically exhibits a compressive strength of 1,000-3,000 psi and tensile strength of 300-1,000 psi. The problems of providing the requisite magnitude of force and power necessary for engagement with and disaggregation of such an environmental threat may be seen to be formidable.
Drilling and operations platforms for use in ice covered areas may take several different forms. One such platform includes a monopod, semi-submersible design utilizing a single rotating cutter completely encircling the intermediate hull section proximate the waterline for ice floe engagement and disaggregation. The cutter is disposed between upper superstructure comprising an operations platform and a submerged hull providing flotation. In this manner, only a relatively narrow profile emerges through encroaching ice layers while platform surface area is maximized and buoyancy size parameters are met, respectively, above and below the ice.
A similar operations platform, which is disclosed in detail in U.S. patent application Ser. No. 772,784, filed Feb. 28, 1977, and entitled "Operations Vessel for Ice Covered Seas", now U.S. Pat. No. 4,102,288, includes a monopod, semi-submersible drilling vessel constructed with an ice breaking bow and aft ice disaggregation apparatus comprising an intermediate hull section. The bow is constructed in the form of a nautical wedge for facilitating transit operation in both open and ice laden waters and for breaking ice within its capability while in the operating mode. In addition, the intermediate hull section also includes a plurality of drums rotatably mounted in generally upstanding relationship relative to the submersible hull. The drums are comprised of an outer surface adapted for breaking, cutting and/or chipping ice engaged thereby. Typically, a pair of drums is mounted for counterrotation such that reaction torque is cancelled.
A floating platform which employs somewhat similar ice engaging and disaggregating means is disclosed in U.S. Pat. No. 4,070,052 entitled "Method and Apparatus for Disaggregating Particulate Matter". The platform disclosed therein includes ice disaggregating apparatus supported by booms which include telescoping struts which support the ice disaggregation apparatus for movement around the entire platform. More particularly, an array of rotatable cutting drums is configured with the axis of rotation of one drum forming an oblique angle with the axis of rotation of a second drum. Rotation of the drums produces improved mass removal effectiveness by first cutting and chipping serrations to form ridges therebetween which subsequently shatter when struck at an oblique angle by the cutters of a second drum. In a variant configuration, an array of three, independently rotatable drums mounted in a triangular configuration is employed. Each drum is comprised of a generally elliptical cross-sectional shape wherein teeth protruding outwardly of adjacent drums do not overlap.
Yet another ice disaggregating system of interest as prior art to the present invention is disclosed in U.S. Pat. No. 4,069,783 entitled "Method and Apparatus for Disaggregating Particulate Matter". As disclosed and discussed in detail therein, the ice disaggregation system comprises the employment of a vertically oriented, rotatable drum disposed ahead of, and adapted to sweep across the path of, a ship from which the drum is supported. The teeth disposed on the rotatable drum have aligned pairs of sledging teeth or progressively increasing lengths extending therefrom. Adjacent each array of aligned pairs of sledging teeth, centrally positioned therebetween and to the rotative rear thereof, there is provided a slugging tooth adapted for striking engagement with the particulate matter engaged and laterally isolated therebetween by the sledging teeth.
It is important to appreciate that the teeth carrying, ice engaging drums discussed in the foregoing are all very large, particularly those associated with the semi-submersible operations vessels. It has been found that, when such drums become so large, the configuration of the ice engaging teeth and their respective mounting means is of great importance if acceptable efficiency is to be obtained in driving the ice disengaging drums and if acceptable tooth life and maintenance methods are to be obtained. That is, the relatively straightforward tooth designs and mounting means characteristic of conventional drum ice disaggregating means (which have been in use for many years) cannot simply be scaled up to obtain the desired quality and economy of operation, fabrication, and maintenance necessary in such very large installations as herein contemplated.